Method for producing 2,3,4,5-tetrachlorobenzotrifluoride

ABSTRACT

According to the invention, 2,3,4,5-tetrachlorobenzotrifluoride is prepared by chlorinating 4-chlorobenzotrifluoride with elementary chlorine. The chlorination is conducted in the presence of 0.1 to 2 wt. % iron (III) chloride and 0.5 to 0.75 wt. % aluminum (III) chloride, each with regard to 4-chlorobenzotrichloride, whereby the weight ratio of iron (III) chloride to aluminum (III) chloride ranges from 10:1 to 1:1.

[0001] The present invention relates to an improved method for producing2,3,4,5-tetrachlorobenzotrifluoride from 4-chlorobenzotrifluoride.

[0002] 2,3,4,5-Tetrachlorobenzotrifluoride is an intermediate for thepreparation of highly effective antibacterial quinolonecarboxylic acidderivatives (see e.g. U.S. Pat. No. 5,599,980).

[0003] A kinetic investigation of the chlorination of4-chlorobenzotrifluoride with chlorine in the presence of iron isreported in Zh. Org. Khimii 20, 2187-2191 (1984). According to this, thetetrachlorobenzotrifluoride formed is virtually only the 2,3,4,5-isomer.A preparative production of 2,3,4,5-tetrachlorobenzotrifluoride is notdescribed therein. In the experimental section of this literaturesource, a chlorination rate of 0.1 g/mol.h is given. For the productionof tetrachlorobenzotrifluoride, that would mean a chlorination time ofmore than 2000 hours, which is much too long for a preparative method.

[0004] A production method for 2,3,4,5-tetrachlorobenzotrifluoride inwhich 4-chlorobenzotrifluoride is reacted with chlorine in the presenceof iron sulfide as catalyst is known from U.S. Pat. No. 5,599,980. Here,the desired product is obtained in a yield of only 56.8% of theory,which is very unsatisfactory.

[0005] A disadvantage of all of the methods known hitherto is theelimination of hydrogen fluoride, which proceeds as a secondaryreaction. This leads to corrosion problems and impairs the quality ofthe hydrochloric acid formed. In the production of3,4-dichlorobenzotrifluoride, an intermediate in the production of2,3,4,5-tetrachlorobenzotrifluoride from 4-chlorobenzotrifluoride, thisproblem was solved by adding calcium chloride to the iron(III) chloridecatalyst (see U.S. Pat. No. 4,401,623). However, such a process cannotbe transferred to the production of 2,3,4,5-tetrachlorobenzotrifluoridesince the absorption of hydrogen fluoride by calcium chloride isreversible (see loc. cit., column 3, lines 14 ff). For the production of2,3,4,5-tetrachlorobenzotrifluoride, higher temperatures are requiredthan for the production of 3,4-dichlorobenzotrifluoride, which favorsthe desorption of hydrogen fluoride. According to U.S. Pat. No.4,401,623, at the minimum temperature of e.g. 80° C. required for theproduction of 2,3,4,5-tetrachlorobenzotrifluoride, the corrosion rate isalready higher by a factor of 5 than at the temperature of 60° C. whichis otherwise used therein (see in particular column 4, Table II).

[0006] There is therefore still a need for a method for producing2,3,4,5-tetrachlorobenzotrifluoride in which shorter reaction times,better yields and reduced contents of hydrogen fluoride in the reactionmixture and in the offgas and thus an improvement in the corrosion rateare possible.

[0007] We have now found a method for producing2,3,4,5-tetrachlorobenzotrifluoride by chlorinating4-chlorobenzotrifluoride with elemental chlorine, which is characterizedin that the chlorination is carried out in the presence of from 0.1 to2% by weight of iron(III) chloride and 0.05 to 0.75% by weight ofaluminum(III) chloride, in each case based on 4-chlorobenzotrifluoride,where the weight ratio of iron(III) chloride to aluminum(III) chlorideis 10:1 to 1:1.

[0008] The method according to the invention can be carried out, forexample, at temperatures in the range 80 to 150° C. Preference is givento working at 100 to 130° C.

[0009] The pressure is not critical for the method according to theinvention. It is possible to work below atmospheric pressure, aboveatmospheric pressure or at atmospheric pressure. Preference is given toworking at atmospheric pressure or at pressures up to 1.5 bar.

[0010] The method according to the invention can be carried out with orwithout solvent additions. Suitable solvents are, in particular,halogenated hydrocarbons. Preference is given to working without solventadditions, i.e. without a diluent.

[0011] The reagents and auxiliaries required for the method according tothe invention can be used in standard commercial grades. The watercontent of the 4-chlorobenzotrifluoride is preferably less than 100 ppm,and the water content of the chlorine is preferably less than 50 ppm.Iron(III) chloride and aluminum(III) chloride can be used with the lowwater contents with which they are available commercially.

[0012] Based on 4-chlorobenzotrifluoride, preference is given to using0.25 to 1% by weight of iron(III) chloride and 0.1 to 0.3% by weight ofaluminum(III) chloride, where the weight ratio of iron(III) chloride toaluminum(III) chloride is 5 to 1.5:1.

[0013] The method according to the invention can be carried out invarious ways, e.g. discontinuously, continuously or semicontinuously inpulses. As an example, in the discontinuous procedure it is possible,for example, to initially introduce 4-chlorobenzotrifluoride, to addiron(III) chloride and aluminum(III) chloride, to heat the mixture, withstirring, to the reaction temperature and to gas with chlorine until thedesired conversion is reached. The gassing with chlorine can be stopped,for example, when the content of 2,3,4,5-tetrachlorobenzotrifluoride inthe reaction mixture is 70 to 80% by weight. The desired product canthen be isolated from the reaction mixture, for example by distillation.Chlorobenzotrifluorides with a lower degree of chlorination are producedhere as forerunnings and can be returned to the chlorination.

[0014] The method according to the invention permits the production of2,3,4,5-tetrachlorobenzotrifluoride in shorter reaction times and withbetter yields. This is surprising since iron(III) chloride andaluminum(III) chloride are often equivalent Lewis acid catalysts. Here,however, they surprisingly interact in a synergistic way.

[0015] It is also surprising that the use according to the invention ofaluminum(III) chloride leads to greatly reduced contents of hydrogenfluoride in the reaction mixture and in the offgas. It is known fromU.S. Pat. No. 5,599,980 (see column 7, line 64 to column 8, line 9) thatit is possible to produce tetrafluorobenzotrichloride fromtetrafluorobenzotrifluoride at temperatures as low as 40° C. usingaluminum(III) chloride. On the basis of this prior art, it is thusexpected that, in the presence of aluminum(III) chloride, the contentsof hydrogen fluoride in the reaction mixture and in the offgas would beincreased, and that corrosion problems caused thereby would be greater.Surprisingly, in the method according to the invention, only lowcontents of hydrogen fluoride arise in the reaction mixture and in theoffgas.

[0016] Finally, in the method according to the invention, it is possibleto isolate from the offgas a hydrochloric acid which can be usedgenerally, whereas in the case of increased contents of hydrogenfluoride, it is only possible to obtain from the offgas a hydrogenfluoride-containing hydrochloric acid which, due to its high corrosionpotential, can only be used for specific purposes.

EXAMPLES Example 1

[0017] 100 parts by weight of 4-chlorobenzotrifluoride were introducedinto a stirred vessel. To this were added 0.5 parts by weight ofiron(III) chloride and 0.167 parts by weight of aluminum(III) chloride.The mixture was heated to 110° C. with stirring and 166 parts by weightof chlorine were introduced, with stirring, over the course of 117hours. After nitrogen had been blown through for 3 hours, 157 parts byweight of a reaction mixture of the following gas chromatographicallyascertained composition were obtained: 0.8% by weight ofdichlorobenzotrifluorides, 13.5% by weight oftrichlorobenzotrifluorides, 74.1% by weight of2,3,4,5-tetrachlorobenzotrifluoride and 8.0% by weight ofpentachlorobenzotrifluoride. This corresponds to a yield of2,3,4,5-tetrachlorobenzotrifluoride of 74.0% of theory. The offgas wasanalyzed for the content of hydrogen fluoride. It was less than 5 ppmbetween the 80th hour and the end of the chlorination. The dichloro- andtrichlorobenzotrifluorides, which are produced with a yield of 1.1 or15.3% of theory, respectively, can be recycled after being separated offby distillation.

Example 2 For Comparison Procedure Only with Iron(III) Chloride asCatalyst

[0018] 100 parts by weight of 4-chlorobenzotrifluoride were introducedinto a stirred vessel. To this were added 0.5 parts by weight ofiron(III) chloride. The mixture was heated to 110° C. with stirring, and166 parts by weight of chlorine were introduced such that a productcomposition was produced which was as similar as possible to that ofExample 1, which required 126 hours. After nitrogen had been blownthrough for 3 hours, 150 parts by weight of a reaction mixture of thefollowing gas chromatographically ascertained composition were obtained:1.2% by weight of dichlorobenzotrifluorides, 13.8% by weight oftrichlorobenzotrifluorides, 73.4% by weight of2,3,4,5-tetrachlorobenzotrifluoride and 7.1% by weight ofpentachlorobenzotrifluoride. This corresponds to a yield of 70.1% oftheory. The offgas was analyzed for the content of hydrogen fluloride.It was between 40 and 50 ppm between the 80th hour and the end of thechlorination.

1. A method for producing 2,3,4,5-tetrachlorobenzotrifluoride bychlorinating 4-chlorobenzotrifluoride with elemental chlorine,characterized in that the chlorination is carried out in the presence offrom 0.1 to 2% by weight of iron(III) chloride and 0.05 to 0.75% byweight of aluminum(III) chloride, in each case based on4-chlorobenzotrifluoride, where the weight ratio of iron(III) chlorideto aluminum(III) chloride is 10:1 to 1:1.
 2. The method as claimed inclaim 1, characterized in that up to is carried out at temperatures inthe range 80 to 150° C.
 3. The method as claimed in claims 1 and 2,characterized in that it is carried out without solvent additions. 4.The method as claimed in claims 1 to 3, characterized in that the watercontent of the 4-chlorobenzotrifluoride used is less than 100 ppm andthe water content of the chlorine used is less than 50 ppm.
 5. Themethod as claimed in claims 1 to 4, characterized in that, based on4-chlorobenzotrifluoride, 0.25 to 1% by weight of iron(III) chloride and0.1 to 0.3% by weight of aluminum(III) chloride are used, where theweight ratio of iron(III) chloride to aluminum(III) chloride is 5 to1.5:1.
 5. The method as claimed in claims 1 to 5, characterized in thatit is carried out discontinuously, continuously or semicontinuously inpulses.